Traditional spark plugs typically include a center wire assembly that longitudinally extends within an insulator axial bore and is responsible for delivering a high voltage ignition pulse from an ignition wire to a single spark gap. The center wire assembly often includes a terminal electrode located towards its upper axial end, a high temperature glass seal and/or suppressive component, and a firing electrode located towards its lower axial end such that it forms a spark gap with an opposing ground electrode.
One example of a prior art spark plug is shown in U.S. Pat. No. 2,969,500, which issued on Jan. 24, 1961 to Andert. The spark plug disclosed in this patent includes a tubular conductor enclosing an insulator and a center electrode. In operation, a distributor directs current from a high voltage coil to various spark plugs in their proper succession. The majority of the spark jumps from the tubular conductor to a ground electrode, while a certain amount also jumps from the center electrode such that it illuminates an associated lamp. The associated lamp indicates that the ignition system is in operation and is working.
Spark plugs having more than one spark gap are also known in the art and include, for instance, U.S. Pat. No. 1,165,492 issued Dec. 28, 1915 to Briggs. This patent teaches a spark plug having two parallel center electrodes extending through separate longitudinal bores in an insulator. One of the center electrodes receives a high voltage ignition pulse from a high tension magneto, while the other one receives a lower voltage ignition pulse from a coil system. One object of the Briggs' invention is to utilize the lower voltage ignition pulse during starting and the high voltage ignition pulse during normal operation.
Another example of a spark plug having more than one spark gap is seen in U.S. Pat. No. 1,229,193 issued Jun. 5, 1917 to Minogue. In that patent, the spark plug has two parallel center electrodes extending through separate longitudinal insulator bores. Each of the center electrodes is radially bent at the firing end, such that in a first embodiment they bend towards each other (solid lines), while in a second embodiment they bend away from each other (phantom lines). The first embodiment acts as a single gap spark plug as one of the electrodes is grounded via connection 15, and the second embodiment acts as a dual gap spark plug as the two electrodes are electrically isolated.
One of the difficulties with dual gap spark plugs of the types described above is that they utilize an asymmetrical insulator which can add significant cost and complexity to the manufacturing process. It is therefore a general object of this invention to provide a dual gap spark plug that permits the use of more standard-shaped insulators.